In my current organisation we must interact with a number of servers with, by modern standards, insecure TLS configuration. In particular, Diffie-Hellman parameters that are too short to be considered secure. OpenSSL and GnuTLS as shipped in Ubuntu 20.04 both refuse to handshake with these services. Well done!

Unfortunately, it’s a large organisation so there is absolutely no chance of getting these server configurations updated. So we need to reconfigure our TLS client libraries to accept the low-security servers.

First, a disclaimer: I am not a cryptographer, security engineer, encryption library developer, informed amateur, or even a particularly observant bystander. You should be accepting security advice from me.

GnuTLS

A great deal of GnuTLS operation is configured using a priority string. In the version I’m looking at, the default priority string is

NORMAL:-VERS-ALL:+VERS-TLS1.3:+VERS-TLS1.2:+VERS-DTLS1.2:%PROFILE_MEDIUM

It’s the %PROFILE_MEDIUM that does things like set the minimum lengths for keys, DH primes, etc. You can find the various security profile options in the GnuTLS manual at Section 6.11 Selecting cryptographic key sizes. My current project needs to support servers using Diffie-Hellman primes of 1024 bits, so I need to use PROFILE_LOW.

You can override the default priority string by editing (or creating) /etc/gnutls/config like so:

[overrides]
default-priority-string = NORMAL:-VERS-ALL:+VERS-TLS1.3:+VERS-TLS1.2:+VERS-DTLS1.2:%PROFILE_LOW

This should then apply to any application that doesn’t specify it’s own priority string.

OpenSSL

Updating the OpenSSL configuration is a much more complicated proposition. The configuration used by applications is stored in a section named by the openssl_conf variable. This is not in a section, so it’ll be at the top of the file if it’s set. In my experience, it often isn’t set.

If it is set, go find the section it names, then follow ssl_conf to the section containing the SSL configuration, then follow system_default to the section it names.

Here, you can specify something like:

CipherString = DEFAULT:@SECLEVEL=1

If all that isn’t already in your openssl.cnf, you need to create a new section, which points to a section, which points to a section. This can all go at the end of the file. Then you need to add a variable not in a section that points to the first of those sections.

Here’s a shell script that does just that:

#!/bin/sh
# Update the OpenSSL configuration to use lower default security level. This
# allows us to connect to TLS servers using insecure certificates issued by the
# internal CA.

set -eux

mv /etc/ssl/openssl.cnf /etc/ssl/openssl.cnf.orig

{
cat <<EOF
# Override the default OpenSSL configuration with less secure settings that
# allow communication with the many services that use insecure certificates
# issued by the internal CA.
openssl_conf = default_conf
EOF

cat /etc/ssl/openssl.cnf.orig;

cat <<EOF
# Default configuration for applications which use OpenSSL.
[ default_conf ]
ssl_conf = ssl_sect

[ ssl_sect ]
system_default = system_default_sect

[ system_default_sect ]
MinProtocol = TLSv1.2
# Be less secure when negotiating ciphers, verifying certificates, etc.
CipherString = DEFAULT:@SECLEVEL=1
EOF
} > /etc/ssl/openssl.cnf

It’s probably a bit too lax but I use it in Docker images based on ubuntu:20.04 and it seems to do the trick.